Tidal encounters of close white dwarf binaries with spinning black holes

TL;DR

Addresses how a close white-dwarf binary on a parabolic orbit around a rapidly spinning intermediate-mass black hole experiences tidal interactions that couple the binary spin, orbital angular momentum, and BH spin. The authors implement a hybrid numerical framework that integrates Kerr geodesics in the spacetime of a Kerr BH with Newtonian SPH hydrodynamics for the binary, fixing a pericentre distance $r_p = 25 r_g$ and considering BH spins $a^\bstar=\pm 0.98$. They find that BH spin imprints distinct tidal dynamics and fallback observables that depend on initial inclination and spin alignment, with prograde encounters often displaying Hills-type disruption and a potential three-hump fallback structure, while retrograde cases tend to preserve bound configurations. These results broaden the understanding of tidal disruption events involving IMBHs and spinning BH backgrounds, with implications for observables such as mass fallback rates and debris dynamics in galactic nuclei.

Abstract

When a stellar binary encounters a spinning black hole, interesting phenomena might result due to the mutual interaction between the binary spin, orbital angular momentum and the black hole spin. Here we consider such encounters between an intermediate mass spinning black hole and a close identical white dwarf binary system whose center of mass follows a parabolic trajectory. After studying a corresponding three-body problem in the point particle approximation, we perform a suite of smoothed particle hydrodynamics based numerical simulations of such scenarios. For this, we integrate the geodesic equations for the spinning black hole, while considering the hydrodynamics and the self and mutual gravitational interactions of the stars in a Newtonian approximation, an approach justified by the choice of parameters in the theory. We consider various initial configurations of the binary center of mass leading to equatorial and off-equatorial orbits, as also various initial inclinations between the binary's initial spin angular momentum and its initial orbital angular momentum. We find that the effects of black hole spin manifest clearly in the tidal dynamics of the binary components, while the observables of tidal encounters such as mass fallback rates are strongly dependent on the initial inclination angle. We show that the influence of the black hole spin emerges in distinct ways for different initial configurations of the binary's spin alignment. We establish that within the ambits of the Hills mechanism, in certain cases, the fallback rate may show a three-hump structure, due to interactions between tidal debris of the individual stars.

Figures (16)

• Figure 1: Qualitative depiction of binary stellar dynamics around a spinning BH.
• Figure 2: Trajectories of binary test particle around the spinning BH. Left Panel: The initial motion of the binary is prograde relative to the OAM ($\vartheta_0=0^\circ$). Right Panel: The initial motion of the binary is retrograde relative to the OAM ($\vartheta_0=180^\circ$).
• Figure 3: Trajectories of binary test particle around the spinning BH whose initial relative angular momenta of binary is subtended by $\vartheta_0=90^\circ$.
• Figure 4: Evolution of the binary spin around a spinning BH. Left Panel: The change in magnitude of the binary spin, $|\boldsymbol{l}_b(t)|$ over normalized time ($t/t_p$, where $t_p=0.0624~\mathrm{hr}$ is the pericentre time) for different initial binary inclinations $\vartheta_0$ around spinning BH ($a^{\star}=-0.98$). There is abrupt gain in the magnitudes of binary spin at $t_p$ from its initial value, $|\boldsymbol{l}_b(0)|=|\mathbf{L}_b|$. The solid lines denote the values of $|\boldsymbol{l}_b(t)|$ in equatorial orbit, $\theta_a=90^\circ$, and dashed lines denotes off-equatorial orbit, $\theta_a=1^\circ$. Right panel: Reorientation of binary spin over time. $\vartheta(t)$ measures the orientation of $\boldsymbol{l}_b$ with respect to $\mathbf{L}_o$. The dotted, dashed, and solid curves correspond to initial binary inclinations of $\vartheta_0 = 0^\circ$, $90^\circ$, and $180^\circ$, respectively.
• Figure 5: Evolution of the binary spin around a spinning BH ($a^{\star}=-0.98$) for initial binary inclinations $\vartheta_0$ and $-\vartheta_0$. Left Panel: Equatorial orbit ($\theta_a=90^\circ$). Right Panel: Off-equatorial orbit ($\theta_a=1^\circ$).